Electrophoretic displays with controlled amounts of pigment

ABSTRACT

An electrophoretic medium has walls defining a microcavity containing an internal phase. This internal phase comprises electrophoretic particles suspended in a suspending fluid and capable of moving therethrough upon application of an electric field to the electrophoretic medium. The average height of the microcavity differs by not more than about 5 μm from the saturated particle thickness of the electrophoretic particle divided by the volume fraction of the electrophoretic particles in the internal phase.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending application Ser.No. 10/837,062, filed Apr. 30, 2004 (Publication No. 2005/0012980),which claims benefit of Provisional Application Ser. No. 60/320,158,filed May 2, 2003, and of Provisional Application Ser. No. 60/320,169,filed May 6, 2003.

This application is also a continuation-in-part of copending applicationSer. No. 11/970,811, filed Jan. 8, 2008, which is a continuation ofcopending application Ser. No. 10/729,044, filed Dec. 5, 2003 (now U.S.Pat. No. 7,352,353, issued Apr. 1, 2008), which is a continuation ofapplication Ser. No. 09/140,860, filed Aug. 27, 1998 (now U.S. Pat. No.6,710,540, issued Mar. 23, 2004). The aforementioned application Ser.No. 09/140,860 is a continuation-in-part of U.S. Ser. No. 08/504,896filed Jul. 20, 1995 (now U.S. Pat. No. 6,124,851, issued Sep. 26, 2000),of U.S. Ser. No. 08/983,404 filed Jul. 19, 1997 (now U.S. Pat. No.7,106,296, issued Sep. 12, 2006), and of U.S. Ser. No. 08/935,800 filedSep. 23, 1997 (now U.S. Pat. No. 6,120,588, issued Sep. 19, 2000), thecontents of all of which are incorporated herein by reference. Theaforementioned application Ser. No. 09/140,860 also claims priority toU.S. Ser. No. 60/057,133 filed Aug. 28, 1997, U.S. Ser. No. 60/057,716,filed Aug. 28, 1997, U.S. Ser. No. 60/057,122, filed Aug. 28, 1997, U.S.Ser. No. 60/057,798, filed Aug. 28, 1997, U.S. Ser. No. 60/057,799 filedAug. 28, 1997, U.S. Ser. No. 60/057,163 filed Aug. 28, 1997, U.S. Ser.No. 60/057,118, filed Aug. 28, 1997, U.S. Ser. No. 60/059,358, filedSep. 19, 1997, U.S. Ser. No. 60/059,543 filed Sep. 19, 1997, U.S. Ser.No. 60/065,529, filed Nov. 18, 1997, U.S. Ser. No. 60/065,630 filed Nov.18, 1997, U.S. Ser. No. 60/065,605 filed Nov. 18, 1997, U.S. Ser. No.60/066,147, filed Nov. 19, 1997, U.S. Ser. No. 60/066,245, filed Nov.20, 1997, U.S. Ser. No. 60/066,246, filed Nov. 20, 1997, U.S. Ser. No.60/066,115 filed Nov. 21, 1997, U.S. Ser. No. 60/066,334 filed Nov. 21,1997, U.S. Ser. No. 60/066,418 filed Nov. 24, 1997, U.S. Ser. No.60/070,940 filed Jan. 9, 1998, U.S. Ser. No. 60/071,371 filed Jan. 15,1998, U.S. Ser. No. 60/072,390 filed Jan. 9, 1998, U.S. Ser. No.60/070,939 filed Jan. 9, 1998, U.S. Ser. No. 60/070,935 filed Jan. 9,1998, U.S. Ser. No. 60/074,454, filed Feb. 12, 1998, U.S. Ser. No.60/076,955 filed Mar. 5, 1998, U.S. Ser. No. 60/076,959 filed Mar. 5,1998, U.S. Ser. No. 60/076,957 filed Mar. 5, 1998, U.S. Ser. No.60/076,978 filed Mar. 5, 1998, U.S. Ser. No. 60/078,363 filed Mar. 18,1998, U.S. Ser. No. 60/083,252 filed Apr. 27, 1998, U.S. Ser. No.60/085,096 filed May 12, 1998, U.S. Ser. No. 60/090,223 filed Jun. 22,1998, U.S. Ser. No. 60/090,232 filed Jun. 22, 1998, U.S. Ser. No.60/092,046 filed Jul. 8, 1998, U.S. Ser. No. 60/092,050 filed Jul. 8,1998, and U.S. Ser. No. 60/093,689 filed Jul. 22, 1998

The entire contents of these patents and copending applications, and ofall other U.S. patents and published and copending applicationsmentioned below, are herein incorporated by reference.

BACKGROUND OF INVENTION

Traditionally, electronic displays such as liquid crystal displays havebeen made by sandwiching an optoelectrically active material between twopieces of glass. In many cases each piece of glass has an etched, clearelectrode structure formed using indium tin oxide. A first electrodestructure controls all the segments of the display that may beaddressed, that is, changed from one visual state to another. A secondelectrode, sometimes called a counter electrode, addresses all displaysegments as one large electrode, and is generally designed not tooverlap any of the rear electrode wire connections that are not desiredin the final image. Alternatively, the second electrode is alsopatterned to control specific segments of the displays. In thesedisplays, unaddressed areas of the display have a defined appearance.

Electrophoretic display media, generally characterized by the movementof particles through an applied electric field, are highly reflective,can be made bistable, and consume very little power. Encapsulatedelectrophoretic displays also enable the display to be printed. Theseproperties allow encapsulated electrophoretic display media to be usedin many applications for which traditional electronic displays are notsuitable, such as flexible displays. The electro-optical properties ofencapsulated displays allow, and in some cases require, novel schemes orconfigurations to be used to address the displays.

Particle-based electrophoretic displays have been the subject of intenseresearch and development for a number of years. In this type of display,a plurality of charged particles move through a suspending fluid underthe influence of an electric field. Electrophoretic displays can haveattributes of good brightness and contrast, wide viewing angles, statebistability, and low power consumption when compared with liquid crystaldisplays. (The terms “bistable” and “bistability” are used herein intheir conventional meaning in the art to refer to displays comprisingdisplay elements having first and second display states differing in atleast one optical property, and such that after any given element hasbeen driven, by means of an addressing pulse of finite duration, toassume either its first or second display state, after the addressingpulse has terminated, that state will persist for at least severaltimes, for example at least four times, the minimum duration of theaddressing pulse required to change the state of the display element. Inpractice, some electrophoretic displays, including some of the displaysof the present invention, are capable of achieving multiple gray states,and, as demonstrated below, are stable not only in their extreme blackand white optical states, but also in their intermediate gray states.Although such displays should properly be described as “multi-stable”rather than “bistable”, the latter term may be used herein forconvenience.) The optical property which is changed by application of anelectric field is typically color perceptible to the human eye, but mayalternatively or in addition, be any one or more of reflectivity,retroreflectivity, luminescence, fluorescence, phosphorescence, or colorin the broader sense of meaning a difference in absorption orreflectance at non-visible wavelengths. Problems with the long-termimage quality of these displays have prevented their widespread usage.For example, particles that make up electrophoretic displays tend tosettle, resulting in inadequate service-life for these displays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspending medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545;6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333;6,704,133; 6,710,540; 6,721,083; 6,724,519; 6,727,881; 6,738,050;6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068;6,825,829; 6,825,970; 6,831,769; 6,839,158; 6,842,167; 6,842,279;6,842,657; 6,864,875; 6,865,010; 6,866,760; 6,870,661; 6,900,851;6,922,276; 6,950,200; 6,958,848; 6,967,640; 6,982,178; 6,987,603;6,995,550; 7,002,728; 7,012,600; 7,012,735; 7,023,420; 7,030,412;7,030,854; 7,034,783; 7,038,655; 7,061,663; 7,071,913; 7,075,502;7,075,703; 7,079,305; 7,106,296; 7,109,968; 7,110,163; 7,110,164;7,116,318; 7,116,466; 7,119,759; 7,119,772; 7,148,128; 7,167,155;7,170,670; 7,173,752; 7,176,880; 7,180,649; 7,190,008; 7,193,625;7,202,847; 7,202,991; 7,206,119; 7,223,672; 7,230,750; 7,230,751;7,236,790; 7,236,792; 7,242,513; 7,247,379; 7,256,766; 7,259,744;7,280,094; 7,304,634; 7,304,787; 7,312,784; 7,312,794; 7,312,916;7,237,511; 7,339,715; 7,349,148; 7,352,353; 7,365,394; and 7,365,733;and U.S. Patent Applications Publication Nos. 2002/0060321;2002/0090980; 2003/0102858; 2003/0151702; 2003/0222315; 2004/0105036;2004/0112750; 2004/0119681; 2004/0155857; 2004/0180476; 2004/0190114;2004/0257635; 2004/0263947; 2005/0000813; 2005/0007336; 2005/0012980;2005/0018273; 2005/0024353; 2005/0062714; 2005/0099672; 2005/0122284;2005/0122306; 2005/0122563; 2005/0134554; 2005/0151709; 2005/0152018;2005/0156340; 2005/0179642; 2005/0190137; 2005/0212747; 2005/0253777;2005/0280626; 2006/0007527; 2006/0038772; 2006/0139308; 2006/0139310;2006/0139311; 2006/0176267; 2006/0181492; 2006/0181504; 2006/0194619;2006/0197737; 2006/0197738; 2006/0202949; 2006/0223282; 2006/0232531;2006/0245038; 2006/0262060; 2006/0279527; 2006/0291034; 2007/0035532;2007/0035808; 2007/0052757; 2007/0057908; 2007/0069247; 2007/0085818;2007/0091417; 2007/0091418; 2007/0109219; 2007/0128352; 2007/0146310;2007/0152956; 2007/0153361; 2007/0200795; 2007/0200874; 2007/0201124;2007/0207560; 2007/0211002; 2007/0211331; 2007/0223079; 2007/0247697;2007/0285385; 2007/0286975; 2007/0286975; 2008/0013155; 2008/0013156;2008/0023332; 2008/0024429; 2008/0024482; 2008/0030832; 2008/0043318;2008/0048969; 2008/0048970; 2008/0054879; 2008/0057252; and2008/0074730; and International Applications Publication Nos. WO00/38000; WO 00/36560; WO 00/67110; and WO 01/07961; and EuropeanPatents Nos. 1,099,207 B1; and 1,145,072 B1.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc.

Hereinafter, the term “microcavity electrophoretic display” will be usedto cover both encapsulated and microcell electrophoretic displays.

Known microcavity electrophoretic displays can be divided into two maintypes, referred to hereinafter for convenience as “single particle” and“dual particle” respectively. A single particle medium has only a singletype of electrophoretic particle suspended in a colored medium, at leastone optical characteristic of which differs from that of the particles.(In referring to a single type of particle, we do not imply that allparticles of the type are absolutely identical. For example, providedthat all particles of the type possess substantially the same opticalcharacteristic and a charge of the same polarity, considerable variationin parameters such as particle size and electrophoretic mobility can betolerated without affecting the utility of the medium.) When such amedium is placed between a pair of electrodes, at least one of which istransparent, depending upon the relative potentials of the twoelectrodes, the medium can display the optical characteristic of theparticles (when the particles are adjacent the electrode closer to theobserver, hereinafter called the “front” electrode) or the opticalcharacteristic of the suspending medium (when the particles are adjacentthe electrode remote from the observer, hereinafter called the “rear”electrode, so that the particles are hidden by the colored suspendingmedium).

A dual particle medium has two different types of particles differing inat least one optical characteristic and a suspending fluid which may beuncolored or colored, but which is typically uncolored. The two types ofparticles differ in electrophoretic mobility; this difference inmobility may be in polarity (this type may hereinafter be referred to asan “opposite charge dual particle” medium) and/or magnitude. When such adual particle medium is placed between the aforementioned pair ofelectrodes, depending upon the relative potentials of the twoelectrodes, the medium can display the optical characteristic of eitherset of particles, although the exact manner in which this is achieveddiffers depending upon whether the difference in mobility is in polarityor only in magnitude. For ease of illustration, consider anelectrophoretic medium in which one type of particles are black and theother type white. If the two types of particles differ in polarity (if,for example, the black particles are positively charged and the whiteparticles negatively charged), the particles will be attracted to thetwo different electrodes, so that if, for example, the front electrodeis negative relative to the rear electrode, the black particles will beattracted to the front electrode and the white particles to the rearelectrode, so that the medium will appear black to the observer.Conversely, if the front electrode is positive relative to the rearelectrode, the white particles will be attracted to the front electrodeand the black particles to the rear electrode, so that the medium willappear white to the observer.

If the two types of particles have charges of the same polarity, butdiffer in electrophoretic mobility (this type of medium may hereinafterto referred to as a “same polarity dual particle” medium), both types ofparticles will be attracted to the same electrode, but one type willreach the electrode before the other, so that the type facing theobserver differs depending upon the electrode to which the particles areattracted. For example suppose the previous illustration is modified sothat both the black and white particles are positively charged, but theblack particles have the higher electrophoretic mobility. If now thefront electrode is negative relative to the rear electrode, both theblack and white particles will be attracted to the front electrode, butthe black particles, because of their higher mobility, will reach itfirst, so that a layer of black particles will coat the front electrodeand the medium will appear black to the observer. Conversely, if thefront electrode is positive relative to the rear electrode, both theblack and white particles will be attracted to the rear electrode, butthe black particles, because of their higher mobility will reach itfirst, so that a layer of black particles will coat the rear electrode,leaving a layer of white particles remote from the rear electrode andfacing the observer, so that the medium will appear white to theobserver: note that this type of dual particle medium requires that thesuspending fluid to sufficiently transparent to allow the layer of whiteparticles remote from the rear electrode to be readily visible to theobserver. Typically, the suspending fluid in such a display is notcolored at all, but some color may be incorporated for the purpose ofcorrecting any undesirable tint in the white particles seentherethrough.

Certain of the aforementioned E Ink and MIT patents and applicationsdescribe electrophoretic media which have more than two types ofelectrophoretic particles within a single capsule. For present purposes,such multi-particle media are regarded as a sub-class of dual particlemedia.

Both single and dual particle electrophoretic displays may be capable ofintermediate gray states having optical characteristics intermediate thetwo extreme optical states already described.

Microcavity electrophoretic displays may have microcavities of anysuitable shape; for example, several of the aforementioned E Ink and MITpatents and applications (see especially U.S. Pat. Nos. 6,067,185 and6,392,785) describe encapsulated electrophoretic displays in whichoriginally-spherical capsules are flattened so that they havesubstantially the form of oblate ellipsoids. When a large number of suchoblate ellipsoidal capsules are deposited upon a substrate, the walls ofthe capsules may contact one another, until the capsules approach aclose-packed condition in which the walls of adjacent capsules areflattened against one another so that the capsules assume substantiallythe form of polygonal prisms. In theory, in a close-packed layer ofcapsules, the individual capsules would have the form of hexagonalprisms, and indeed micrographs of some encapsulated electrophoreticmedia show a close approach to this condition. However, more typicallythe individual capsules have substantially the form of irregularpolygonal prisms. In polymer-dispersed encapsulated electrophoreticmedia, there are of course no individual capsules, but the droplets ofinternal phase may assume forms similar to the capsule forms alreadydiscussed.

Thus, microcavities in microcavity electrophoretic displays may beirregular. The following discussion will consider microcavities in alaminar film having substantial dimensions in a plane considered ashaving X and Y axes, and a much smaller dimension perpendicular to thisplane, this dimension being denoted the Z axis. The average internalheight of the microcavity along the Z axis will be denoted the “internalphase height” or “IP height” of the microcavity. The average areaparallel to the XY plane of the microcavity (averaged along the Z axis)excluding capsule or cavity walls will be denoted the “IP area”, whilethe corresponding average area including the capsule or cavity wallswill be denoted the “capsule area”. The maximum diameter parallel to theXY plane of the microcavity at any height excluding capsule or cavitywalls will be denoted the “IP diameter”, while the corresponding averagediameter including the capsule or cavity walls will be denoted the“capsule diameter”.

It has long been known that, to optimize the optical performance ofelectrophoretic and other electro-optic displays, it is desirable tomaximize the active fraction of the display area, i.e., the fraction ofthe display area which can change optical state when an electric fieldis applied to the electro-optic medium. Inactive areas of the display,such as the black masks often used in liquid crystal displays, and thearea occupied by capsule or microcavity walls in microcavityelectrophoretic displays, do not change optical state when an electricfield is applied, and hence reduce the contrast between the extremeoptical states of the display. However, there is relatively littleconsideration in the published literature relating to other parametersaffecting the optical performance of electrophoretic displays, and inparticular the amount of pigment needed in the electrophoretic medium.This may be due, in part, to the fact that most electrophoretic displaysdiscussed in the literature have been single particle electrophoreticdisplays, and in such displays the limiting factor on the thickness ofthe electrophoretic medium is normally the optical density of the dye inthe suspending fluid, and not the amount of pigment present. This is notthe case with dual particle electrophoretic displays, and may not be thecase with single particle displays using dyes with optical densitieshigher than those used in most prior art electrophoretic displays.

It has now been found that the optical performance of electrophoreticdisplays is substantially affected by variations in the amount ofpigment present in the electrophoretic medium, the IP height of themedium, and the pigment loading of the internal phase (i.e., theproportion of the volume of the internal phase which is comprised ofpigment), and this invention relates to electrophoretic media anddisplays in which the relationships among these various parameters arecontrolled so as to improve, and desirably to optimize, the opticalperformance of the media and displays.

SUMMARY OF INVENTION

Accordingly, this invention provides an electrophoretic medium havingwalls defining at least one microcavity containing an internal phase,this internal phase comprising a plurality of at least one type ofelectrophoretic particle suspended in a suspending fluid and capable ofmoving therethrough upon application of an electric field to theelectrophoretic medium, the average height of the at least onemicrocavity differing by not more than about 5 μm from the saturatedparticle thickness of the electrophoretic particle divided by the volumefraction of the electrophoretic particles in the internal phase.

The term “saturated particle thickness” of electrophoretic particles ina microcavity is used herein to denote the thickness of the layer ofparticles which would be formed over the IP area of the microcavityusing an internal phase containing just sufficient electrophoreticparticles that, if application of a specific electric field to themedium for a time T suffices to switch the electrophoretic mediumbetween its extreme optical states, variations in the time ofapplication of this specific electric field within the range of 0.95 to1.05 T will not change the optical properties of either extreme state ofthe electrophoretic medium by more than 2 units of L*, where L* has theusual CIE definition. This saturated particle thickness is calculatedwithout regard to packing factors; in other words, the saturatedparticle thickness is the hypothetical thickness of the layer whichwould be formed over the IP area if the electrophoretic particles formeda completely solid layer, without voids, over this area. For example, ifan electrophoretic medium has an IP height of 50 μm and contains 10percent by volume of electrophoretic particles, its saturated particlethickness is 5 μm. As will readily be apparent to those familiar withthe packing of multi-particle layers, this thickness does not correspondto the actual thickness of the layer of particles formed when all theparticles are driven to one end surface of the microcavity, sinceinevitably this particle layer will contain a substantial volumefraction of voids. For the sake of simplicity, suppose theelectrophoretic medium comprises spherical particles of essentiallyuniform diameter which form an essentially close-packed layer. Since thepacking fraction for close-packed uniform spheres is approximately 0.64,the actual thickness of the layer formed on one end surface of themicrocavity will be about 5/0.64 or 7.8 μm.

In preferred forms of the present invention, variations in the time ofapplication of this specific electric field within the range of 0.9 to1.1 T will not change the optical properties of either extreme state ofthe electrophoretic medium by more than 2 units of L*, and in especiallypreferred forms of the invention variations in the time of applicationof this specific electric field within the range of 0.8 to 1.2 T willnot change the optical properties of either extreme state of theelectrophoretic medium by more than 2 units of L*.

The saturated particle thickness is typically between about 1 and about5 μm, and desirably between about 1.5 and about 2.5 μm. The volumefraction of electrophoretic particles in the internal phase (i.e., thefraction of the volume of the internal phase occupied by theelectrophoretic particles) is typically from 3 to 40 percent, anddesirably in the range to 6 to 18 percent.

The electrophoretic medium of the present invention may be of any of thetypes described above. Thus, the electrophoretic medium may be a singleparticle medium comprising a single type of electrophoretic particle ina colored suspending fluid. Alternatively, the electrophoretic mediummay be a dual particle medium comprising a first type of electrophoreticparticle having a first optical characteristic and a firstelectrophoretic mobility and a second type of electrophoretic particlehaving a second optical characteristic different from the first opticalcharacteristic and a second electrophoretic mobility different from thefirst electrophoretic mobility. In such a dual particle medium, thesuspending fluid may be uncolored. The electrophoretic medium may be ofthe microcell type, in which the electrophoretic particles and thesuspending fluid are retained within a plurality of cavities formedwithin a carrier medium. Alternatively, the electrophoretic medium maybe an encapsulated electrophoretic medium, in which the electrophoreticparticles and the suspending fluid are held within a plurality ofcapsules.

One type of display in which the present invention may be especiallyuseful is the so-called “shutter mode” microcavity display. A shuttermode microcavity display is a microcavity display having one “opaque”optical state in which the display (or any given pixel thereof) displaysthe color or other optical characteristic of an electrophoreticparticle, and a second optical state in the which the electrophoreticmedium or pixel thereof is light-transmissive. Such a shutter modedisplay may be of the single or dual particle type, and may have morethan the two specified optical states; for example, a dual particleshutter mode display using black and white electrophoretic particles,may have a black opaque state, a white opaque state and alight-transmissive state. The light-transmissive state of a shutter modedisplay is typically produced by confining the electrophoretic particlesin a minor proportion of the cross-sectional area of each microcavity sothat light is free to pass through the major proportion of thiscross-sectional area. The confinement of the electrophoretic particlesto the minor proportion of the cross-sectional area may be effected byusing a shaped microcavity (see, for example, the aforementioned U.S.Pat. Nos. 6,130,774 and 6,172,798), by placement of electrodes inspecific positions relative to the microcavity (see, for example, theaforementioned U.S. Pat. No. 7,170,670 and Japanese Published PatentApplications Nos. 2002-174828 and 2001-356374), or by dielectrophoreticdriving of the electrophoretic particles (see, for example, U.S. Pat.No. 7,259,744).

Although the second optical state of a shutter mode display has beenreferred to above as “light-transmissive”, a shutter mode display mayincorporate a colored or uncolored reflector adjacent the microcavitymedium and on the opposed side thereof from that normally by an observer(this opposed surface hereinafter for convenience being referred to asthe “rear surface” of the microcavity medium) so that (as described forexample in the aforementioned U.S. Pat. No. 7,259,744) thelight-transmissive optical state of the display actually displays thecolor (if any) of the reflector. In particular, an advantageous form ofcolor microcavity display may be formed by providing a backplane havinga plurality of pixel electrodes, forming a color filter or reflector onthe backplane, and then forming a layer of a shutter mode microcavitymedium over the color filter or reflector. A microcell medium might beformed by photolithographic techniques, by forming a layer ofphotoresist over the color filter, and exposing and developing in theconventional manner to form cells walls separating a plurality ofmicrocells. Alternatively, a layer, typically a polymer layer, might beprovided over the color filter and microcavities formed mechanicallytherein, or a preformed layer containing microcavities provided over thecolor filter. In either case, the microcavities formed can be filledwith an electrophoretic mixture (electrophoretic particles plussuspending fluid) and sealed.

Regardless of the exact method used for its manufacture and the exacttype of electrophoretic medium employed, this type of color shutter modedisplay has the advantages that positioning the color filter withrespect to the pixel electrodes is simplified, since the pixelelectrodes are readily visible during formation or attachment of thefilter, and, more importantly, that the positioning of the color filteradjacent the pixel electrodes avoids visible artifacts which may occurdue to parallax when a color filter substantially separated from abackplane (for example, a color filter on the opposed side of theelectrophoretic medium from the backplane) is viewed off-axis.

One problem with such shutter mode microcavity displays is ensuring goodcontrast ratio, since even in the light-transmissive optical state ofsuch a shutter mode display, the minor proportion of each microcavityoccupied by the electrophoretic particles still displays the color ofthose particles (or a mixture of the relevant colors, in the case of adual particle display), and this continuing display of the color of theelectrophoretic particles reduces the contrast ratio. The presentinvention enables one to control the amount of electrophoretic particlesneeded in a microcavity display, thus minimizing the proportion of eachmicrocavity occupied by the electrophoretic particles in thelight-transmissive state of the display and maximizing the contrastratio, while still providing sufficient electrophoretic particles toensure good optical properties in the first optical state of thedisplay.

A preferred white electrophoretic particle for use in the presentelectrophoretic media comprises titania (TiO₂). If the electrophoreticmedium is of the dual particle type, it may further comprise darkcolored particles formed from carbon black or copper chromite, the darkcolored particles formed from carbon black or copper chromite and havingan electrophoretic mobility different from the electrophoretic mobilityof the titania particles.

Useful embodiments of the present invention may have an IP heightbetween about 10 and about 30 μm and a volume fraction ofelectrophoretic particles of between about 3 and about 15 percent.Preferred embodiments have an IP height between about 12 and about 25 μmand a volume fraction of electrophoretic particles of between about 5and about 12 percent. The viscosity of the internal phase is typicallyless than about 5 mPa sec, and typically greater than about 1 mPa sec.

This invention extends to an electrophoretic display comprising anelectrophoretic medium of the present invention and at least oneelectrode disposed adjacent the electrophoretic medium and arranged toapply an electric field thereto. Typically, such an electrophoreticdisplay will have a rear electrode structure having a plurality ofelectrodes arranged to apply an electric field to the electrophoreticmedium.

In another aspect, this invention provides an electrophoretic suspensionintended for use in an electrophoretic display and comprising more thanabout 5 percent by weight of white particles, the suspension having aviscosity of from about 2 to about 7 mPa sec.

This electrophoretic suspension of the present invention may include anyof the preferred features of the electrophoretic medium of the presentinvention, as already described.

The present invention can provide a highly-flexible, reflective displaywhich can be manufactured easily, consumes little (or no in the case ofbistable displays) power, and can, therefore, be incorporated into avariety of applications. The invention features a printable displaycomprising an encapsulated electrophoretic display medium. The resultingdisplay is flexible. Since the display media can be printed, the displayitself can be made inexpensively.

An encapsulated electrophoretic display can be constructed so that theoptical state of the display is stable for some length of time. Thedefinition of a bistable state depends on the application for thedisplay. A slowly-decaying optical state can be effectively bistable ifthe optical state is substantially unchanged over the required viewingtime. For example, in a display which is updated every few minutes, adisplay image which is stable for hours or days is effectively bistablefor that application. In this application, the term bistable alsoindicates a display with an optical state sufficiently long-lived as tobe effectively bistable for the application in mind. Alternatively, itis possible to construct encapsulated electrophoretic displays in whichthe image decays quickly once the addressing voltage to the display isremoved (i.e., the display is not bistable or multistable). As will bedescribed, in some applications it is advantageous to use anencapsulated electrophoretic display which is not bistable. Whether ornot an encapsulated electrophoretic display is bistable, and its degreeof bistability, can be controlled through appropriate chemicalmodification of the electrophoretic particles, the suspending fluid, thecapsule, and binder materials.

An encapsulated electrophoretic display may take many forms. The displaymay comprise capsules dispersed in a binder. The capsules may be of anysize or shape. The capsules may, for example, be spherical and may havediameters in the millimeter range or the micron range, but is preferablyfrom ten to a few hundred microns. The capsules may be formed by anencapsulation technique, as described below. Particles may beencapsulated in the capsules. The particles may be two or more differenttypes of particles. The particles may be colored, luminescent,light-absorbing or transparent, for example. The particles may includeneat pigments, dyed (laked) pigments or pigment/polymer composites, forexample. The display may further comprise a suspending fluid in whichthe particles are dispersed.

The successful construction of an encapsulated electrophoretic displayrequires the proper interaction of several different types of materialsand processes, such as a polymeric binder and, optionally, a capsulemembrane. These materials must be chemically compatible with theelectrophoretic particles and fluid, as well as with each other. Thecapsule materials may engage in useful surface interactions with theelectrophoretic particles, or may act as a chemical or physical boundarybetween the fluid and the binder.

As already indicated, in some cases, the encapsulation step of theprocess is not necessary, and the electrophoretic fluid may be directlydispersed or emulsified into the binder (or a precursor to the bindermaterials) and an effective “polymer-dispersed electrophoretic display”constructed. In such displays, voids created in the binder may bereferred to as capsules or microcapsules even though no capsule membraneis present. The binder dispersed electrophoretic display may be of theemulsion or phase separation type.

Throughout the specification, reference will be made to printing orprinted. As used throughout the specification, printing is intended toinclude all forms of printing and coating, including: premeteredcoatings such as patch die coating, slot or extrusion coating, slide orcascade coating, and curtain coating; roll coating such as knife overroll coating, forward and reverse roll coating; gravure coating; dipcoating; spray coating; meniscus coating; spin coating; brush coating;air knife coating; silk screen printing processes; electrostaticprinting processes; thermal printing processes; and other similartechniques. A “printed element” refers to an element formed using anyone of the above techniques.

This invention provides novel methods and apparatus for controlling andaddressing particle-based displays. Additionally, the inventiondiscloses applications of these methods and materials on flexiblesubstrates, which are useful in large-area, low cost, or high-durabilityapplications.

In one aspect, the present invention relates to an encapsulatedelectrophoretic display. The display includes a substrate and at leastone capsule containing a highly-resistive fluid and a plurality ofparticles. The display also includes at least two electrodes disposedadjacent the capsule, a potential difference between the electrodescausing some of the particles to migrate toward at least one of the twoelectrodes. This causes the capsule to change optical properties.

In another aspect, the present invention relates to a coloredelectrophoretic display. The electrophoretic display includes asubstrate and at least one capsule containing a highly-resistive fluidand a plurality of particles. The display also includes coloredelectrodes. Potential differences are applied to the electrodes in orderto control the particles and present a colored display to a viewer.

In yet another aspect, the present invention relates to anelectrostatically addressable display comprising a substrate, anencapsulated electrophoretic display adjacent the substrate, and anoptional dielectric sheet adjacent the electrophoretic display.Application of an electrostatic charge to the dielectric sheet ordisplay material modulates the appearance of the electrophoreticdisplay.

In still another aspect, the present invention relates to anelectrostatically addressable encapsulated display comprising a film anda pair of electrodes. The film includes at least one capsule containingan electrophoretic suspension. The pair of electrodes is attached toeither side of the film. Application of an electrostatic charge to thefilm modulates the optical properties.

In still another aspect, the present invention relates to anelectrophoretic display that comprises a conductive substrate, and atleast one capsule printed on such substrate. Application of anelectrostatic charge to the capsule modulates the optical properties ofthe display.

In still another aspect the present invention relates to a method formatrix addressing an encapsulated display. The method includes the stepof providing three or more electrodes for each display cell and applyinga sequence of potentials to the electrodes to control movement ofparticles within each cell.

In yet another aspect, the present invention relates to a matrixaddressed electrophoretic display. The display includes a capsulecontaining charged particles and three or more electrodes disposedadjacent the capsule. A sequence of voltage potentials is applied to thethree or more electrodes causing the charged particles to migrate withinthe capsule responsive to the sequence of voltage potentials.

In still another aspect, the present invention relates to a rearelectrode structure for electrically addressable displays. The structureincludes a substrate, a first electrode disposed on a first side of thesubstrate, and a conductor disposed on a second side of the substrate.The substrate defines at least one conductive via in electricalcommunication with both the first electrode and the conductor.

BRIEF DESCRIPTION OF DRAWINGS

The invention is pointed out with particularity in the appended claims.The advantages of the invention described above, together with furtheradvantages, may be better understood by referring to the followingdescription taken in conjunction with the accompanying drawings. In thedrawings, like reference characters generally refer to the same partsthroughout the different views. Also, the drawings are not necessarilyto scale, emphasis instead generally being placed upon illustrating theprinciples of the invention.

FIG. 1A is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich the smaller electrode has been placed at a voltage relative to thelarge electrode causing the particles to migrate to the smallerelectrode.

FIG. 1B is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich the larger electrode has been placed at a voltage relative to thesmaller electrode causing the particles to migrate to the largerelectrode.

FIG. 1C is a diagrammatic top-down view of one embodiment of arear-addressing electrode structure.

FIG. 2A is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerassociated with the larger electrode in which the smaller electrode hasbeen placed at a voltage relative to the large electrode causing theparticles to migrate to the smaller electrode.

FIG. 2B is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerassociated with the larger electrode in which the larger electrode hasbeen placed at a voltage relative to the smaller electrode causing theparticles to migrate to the larger electrode.

FIG. 2C is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerdisposed below the larger electrode in which the smaller electrode hasbeen placed at a voltage relative to the large electrode causing theparticles to migrate to the smaller electrode.

FIG. 2D is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerdisposed below the larger electrode in which the larger electrode hasbeen placed at a voltage relative to the smaller electrode causing theparticles to migrate to the larger electrode.

FIG. 3A is a diagrammatic side view of an embodiment of an addressingstructure in which a direct-current electric field has been applied tothe capsule causing the particles to migrate to the smaller electrode.

FIG. 3B is a diagrammatic side view of an embodiment of an addressingstructure in which an alternating-current electric field has beenapplied to the capsule causing the particles to disperse into thecapsule.

FIG. 3C is a diagrammatic side view of an embodiment of an addressingstructure having transparent electrodes, in which a direct-currentelectric field has been applied to the capsule causing the particles tomigrate to the smaller electrode.

FIG. 3D is a diagrammatic side view of an embodiment of an addressingstructure having transparent electrodes, in which an alternating-currentelectric field has been applied to the capsule causing the particles todisperse into the capsule.

FIG. 4A is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich multiple smaller electrodes have been placed at a voltage relativeto multiple larger electrodes, causing the particles to migrate to thesmaller electrodes.

FIG. 4B is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich multiple larger electrodes have been placed at a voltage relativeto multiple smaller electrodes, causing the particles to migrate to thelarger electrodes.

FIG. 5A is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display havingcolored electrodes and a white electrode, in which the coloredelectrodes have been placed at a voltage relative to the white electrodecausing the particles to migrate to the colored electrodes.

FIG. 5B is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display havingcolored electrodes and a white electrode, in which the white electrodehas been placed at a voltage relative to the colored electrodes causingthe particles to migrate to the white electrode.

FIG. 6 is a diagrammatic side view of an embodiment of a color displayelement having red, green, and blue particles of differentelectrophoretic mobilities.

FIGS. 7A-7B depict the steps taken to address the display of FIG. 6 todisplay red.

FIGS. 8A-8D depict the steps taken to address the display of FIG. 6 todisplay blue.

FIGS. 9A-9C depict the steps taken to address the display of FIG. 6 todisplay green.

FIG. 10 is a perspective embodiment of a rear electrode structure foraddressing a seven segment display.

FIG. 11 is a perspective embodiment of a rear electrode structure foraddressing a three by three matrix display element.

FIG. 12 is a cross-sectional view of a printed circuit board used as arear electrode addressing structure.

FIG. 13 is a cross-sectional view of a dielectric sheet used as a rearelectrode addressing structure.

FIG. 14 is a cross-sectional view of a rear electrode addressingstructure that is formed by printing.

FIG. 15 is a perspective view of an embodiment of a control gridaddressing structure.

FIG. 16 is an embodiment of an electrophoretic display that can beaddressed using a stylus.

DETAILED DESCRIPTION

As already indicated, it has been found that, in microcavityelectrophoretic displays, there is an optimum IP height related to twokey variables, namely the saturated particle thickness of theelectrophoretic particles, i.e., the minimum thickness of each pigmentto achieve an adequate optical state, and the volume fraction of thatpigment in the internal phase of the display.

At first glance, it might appear that achieving an “adequate opticalstate” in an electrophoretic display is solely a function of the desiredoptical property of the pigments used for any given application.However, it has been found that if an electrophoretic medium does notcontain sufficient pigment, the optical properties of the medium may beadversely affected; for example, if an electrophoretic medium containsinsufficient white pigment, the reflectivity of the white state of themedium may be lower than the same state of a similar medium containingmore white pigment.

For various technical reasons, it is generally desirable to keep anelectrophoretic medium as thin as possible consistent with good opticalproperties. Since the rate at which electrophoretic particles move isdetermined by electric field strength, and since (all other factorsbeing equal) the electric field strength in an electrophoretic displayis proportional to the voltage applied between the electrodes divided bythe distance between these electrodes, it is generally desirable to keepthis distance to a minimum (i.e., to keep the electrophoretic medium asthin as possible) in order to keep the operating voltage as low aspossible, a low operating voltage being desirable to reduce energyconsumption by the display (especially in portable, battery-drivendevices) and to minimize the cost and complexity of electronic circuitryneeded to drive the display. Also, keeping the electrophoretic medium asthin as possible reduces the distance which the electrophoreticparticles need to travel during switching of the display between itsextreme optical states and thus, at a constant electric field, increasesthe switching speed of the display. Also, in certain applications,electrophoretic displays are attractive because they can be madeflexible, and it is easier to produce a flexible display with a thinelectrophoretic medium. Hence, it might at first glance appear that anelectrophoretic display should have a minimum IP height and a highvolume fraction of pigment in the internal phase so as to providesufficient pigment to ensure an optimum optical state when that pigmentis visible.

However, there are some countervailing considerations. Increased pigmentloading will typically result in higher viscosity of the internal phase,and this higher viscosity reduces electrophoretic particle velocity andslows the switching speed of the display for a given applied electricfield.

Thus, the optimum formulation of an electrophoretic medium for anyparticular combination of pigment(s), suspending fluid, operatingvoltage and desired switching time is a complicated matter. Thesituation is further complicated by the complex relationships betweenapplied voltages and optical states in electrophoretic media. Asdiscussed in the aforementioned U.S. Pat. No. 7,012,600 and severalother of the aforementioned E Ink and MIT patents and applications,electrophoretic media do not act as simple voltage transducers (as doliquid crystals) but rather act, to a first approximation, as impulsetransducers, so that the final state of a pixel depends not only uponthe electric field applied but also upon the state of the pixel prior tothe application of the electric field. This type of behavior can causeserious complications when it is desired to produce an area ofsupposedly uniform color on a display. Consider, for example, a blackand white display intended for use in reading black text, with orwithout illustrations, on a white background. When such a display isre-written (i.e., when a new page is displayed), unless both theelectrophoretic medium formulation and the drive scheme employed arecarefully chosen, there may be small variations among the optical statesof the numerous pixels in the supposedly uniform white background, andthe human eye is very sensitive to such small variations in opticalstates in a supposedly uniform area, especially since readers areaccustomed to a highly uniform white background on a printed page.

In accordance with the present invention it has been found that, tosecure good optical performance from a microcavity electrophoreticmedium, it is important to correlate the average height of themicrocavities with the saturated particle thickness (as defined above)and the volume fraction of the electrophoretic particles in the internalphase of the electrophoretic medium. It has been found that there is anoptimum IP height for microcavity electrophoretic media. If thesaturation thickness of a pigment is T and the volume fraction of thepigment in the internal phase is F, the optimum IP height is T/F, and inpractice it is desirable that the actual IP height not differ from thisoptimum value by more than about 5 μm. When an electrophoretic mediumcontains two or more pigments, the value of T/F should be calculatedseparately for each pigment, and the optimum IP height set to thelargest of the resultant values.

It will readily be apparent that, in an electrophoretic mediumcontaining more than one type of electrophoretic particle, each type ofparticle will have its own saturated particle thickness. Single particleelectrophoretic media typically comprise a white pigment in a dyedsuspending fluid, while dual particle electrophoretic media typicallycomprise white and black particles in an uncolored suspending fluid.However, in both cases, the critical saturated particle thickness isusually that of the white particles, since the white particles scatterlight while the black particles absorb it, and the pigment thicknessneeded to scatter light is greater than that required to absorb light.

It has been found that, if the microcavities in an electrophoreticmedium have an IP height significantly greater than this optimum T/Fvalue, display performance is reduced. One reason is that the distancethat the pigment must travel in order to reach the microcavity wall isgreater. A second reason is that for a given voltage field across theinternal phase, the field strength is reduced. A low field strengthreduces particle velocity. Furthermore, in a multi-pigment system inwhich particles of opposite charge may have a tendency to aggregate, alow field strength reduces the number of aggregates that are separated.On the other hand, if the microcavities have an IP height significantlyless than this optimum value, the desired optical state may not beachieved due to insufficient optical density of pigment.

To evaluate various electrophoretic media, one can measure the totalpigment “saturation thickness”, achieved from a pulse length time andelectric field level across an internal phase, under which a change inthe pulse period would change the optical properties of the pigment byno more than a desired threshold amount for visual artifacts. In atypical system with a typical white/black switching speed around 300 ms,at a saturated thickness a pulse length change of 50 ms at 15V wouldchange the optical properties by less than 2 L*. In systems employingfaster switching electrophoretic media, a saturated thickness may beadequate if the optical properties would change by less than 2 L* forpulse length variation about 5-20% of the typical white/black switchspeed of the medium. As already mentioned, for purposes of cleardefinition, the term “saturated particle thickness” of electrophoreticparticles in a microcavity is used herein to denote the thickness(assuming 100 percent packing) of the layer of particles which would beformed over the IP area of the microcavity using an internal phasecontaining just sufficient electrophoretic particles that, ifapplication of a specific electric field to the medium for a time Tsuffices to switch the electrophoretic medium between its extremeoptical states, variations in the time of application of this specificelectric field within the range of 0.95 to 1.05 T (i.e., variations intime of ±5 percent) will not change the optical properties of eitherextreme state of the electrophoretic medium by more than 2 units of L*.Desirably, the system should withstand variations in time of ±10percent, and preferably ±20 percent, without changes in opticalproperties exceeding 2 L*.

A preferred white pigment for use in electrophoretic media is titania.The titania desirably has a surface coating of silica and/or alumina,and is also desirably polymer coated, as described in the aforementionedU.S. Pat. No. 6,822,782 or in the related U.S. Pat. No. 7,230,750. As isknown in the art of pigments and paints, the titania particles shoulddesirable be between 0.1 μm and 0.5 μm in diameter, and ideally between0.2 μm and 0.4 μm in diameter, for greatest efficiency in scatteringwith minimal thickness. However a composite particle may also be usedthat contains multiple pigment particles. Titania particles, especiallythose described in the aforementioned U.S. Pat. Nos. 6,822,782 and7,230,750, can have saturated particle thicknesses in the range of fromabout 1 to 10 μm, and desirably from about 1 to about 5 μm, dependingsomewhat upon the addressing waveform used. The present inventors havefound that, in one preferred titania/carbon black dual particleelectrophoretic medium of the present invention, using a preferredaddressing pulse of 15 V and a pulse length of between 200 and 500 ms,the titania provided adequate coverage levels at thicknesses between 1.5and 2.5 μm. In another preferred electrophoretic medium having a lowerviscosity, faster switching internal phase driven by 15 V pulses with apulse length of 100 ms, the titania also provided adequate coverage atthicknesses between 1.5 and 2.5 μm.

Copper chromite particles may be used in place of carbon black particlesas the dark colored particles in dual particles media of the presentinvention (or, indeed, in single particle media where a dark particle isdesired). The preparation and use of copper chromite particles inelectrophoretic media is fully described in U.S. Pat. No. 7,002,728.

As discussed above, at first glance it appears desirable to formulatethin electrophoretic media with high pigment loadings, but the abilityto do so is limited by the increase in viscosity associated with highpigment loadings. Some increase in pigment loading may be advantageous,as compared with pigment loadings used in prior art electrophoreticdisplays. For example, to achieve whiter systems, it can be moderatelyuseful to increase titania loading to higher levels such as 5-7 μmthickness. Also, whereas some electrophoretic suspensions known in theart have employed a pigment loading of less than 2 percent by weight, ithas been found that an internal phase comprising up to 45 percent byweight or 15 percent by volume of titania particles can have a viscositythat permits the particles to achieve an adequate velocity underelectric fields of a strength useful in commercial devices, so as toenable the devices to use driving voltages typically 15 V or less.Preferred media of the present invention may typically have a titanialoading of 5 to 15 percent by volume with an internal phase viscositybetween 1 and 6 mPa sec. Given a saturated particle thickness of 1.5 to2.5 μm, and a titania loading of 10 percent by volume, it has been foundthat visual artifacts are reduced when the IP height for a microcavityis between 15 μm and 25 μm, with the optimum value being substantially20 μm.

Other types of internal phases may permit reduced viscosity, thuspermitting a higher pigment loading. For example, an internal phaseusing a gaseous suspending fluid (see, for example, 2004/0112750) wouldbe able to support a much higher pigment loading and correspondingly alower IP height. Such gas-based phases could function with particleloadings as high as 90 percent by volume.

It is believed (although the invention is in no way limited by thisbelief) that one of the reasons for the improved optical states achievedby the present invention is that if, in a microcavity electrophoreticdisplay, the pigment is not sufficiently thick, the display isvulnerable to image ghosting. The reasons for such image ghosting mayinclude small voltage variations in the addressing system,slowly-decaying remnant voltages or polarization in the microcavities ofthe display, settling of the pigments over time, improper mixing ofvarious pigments, and differences between the RC time constant of theinternal phase and its external environment, including any binderpresent. All of these effects can cause variations in the amount ofpigment visible to an observer, superimposed on the variations intendedto be caused by the addressing of the display.

The “visual artifact level” of a display (typically a high resolutiondisplay) may be measured by any suitable means. In one method, manypixels are each subjected to a different switching history typical ofthe intended usage model. The greatest optical difference between anytwo pixels is the “maximum visual artifact level.” Alternatively, asingle pixel may be subjected to many different switching histories anda consistent test addressing pulse then applied. The greatest opticaldifference between the resulting optical states is another way tomeasure the “maximum visual artifact level.”

To achieve consistent image quality with minimal visual artifacts, it isdesirable that the electrophoretic medium contain a minimum adequatethickness of the pigment such that a small variation in pigment levelhas a minimal optical effect. For portable high-resolution displayapplications, this optical effect should ideally be no more than 1 to 2L* units, given typical variations in actual pigment packingthicknesses.

Having described in detail the features peculiar to the “controlledpigment” media and displays of the present invention, more generalguidance regarding the construction and uses of electrophoretic mediawill now be given.

An electronic ink is an optoelectronically active material whichcomprises at least two phases: an electrophoretic contrast media phaseand a coating/binding phase. The electrophoretic phase comprises, insome embodiments, a single species of electrophoretic particlesdispersed in a clear or dyed medium, or more than one species ofelectrophoretic particles having distinct physical and electricalcharacteristics dispersed in a clear or dyed medium. In some embodimentsthe electrophoretic phase is encapsulated, that is, there is a capsulewall phase between the two phases. The coating/binding phase includes,in one embodiment, a polymer matrix that surrounds the electrophoreticphase. In this embodiment, the polymer in the polymeric binder iscapable of being dried, crosslinked, or otherwise cured as intraditional inks, and therefore a printing process can be used todeposit the electronic ink onto a substrate. An electronic ink iscapable of being printed by several different processes, depending onthe mechanical properties of the specific ink employed. For example, thefragility or viscosity of a particular ink may result in a differentprocess selection. A very viscous ink would not be well-suited todeposition by an inkjet printing process, while a fragile ink might notbe used in a knife over roll coating process.

The optical quality of an electronic ink is quite distinct from otherelectronic display materials. The most notable difference is that theelectronic ink provides a high degree of both reflectance and contrastbecause it is pigment based (as are ordinary printing inks). The lightscattered from the electronic ink comes from a very thin layer ofpigment close to the top of the viewing surface. In this respect itresembles an ordinary, printed image. Also, electronic ink is easilyviewed from a wide range of viewing angles in the same manner as aprinted page, and such ink approximates a Lambertian contrast curve moreclosely than any other electronic display material. Since electronic inkcan be printed, it can be included on the same surface with any otherprinted material, including traditional inks. Electronic ink can be madeoptically stable in all display configurations, that is, the ink can beset to a persistent optical state. Fabrication of a display by printingan electronic ink is particularly useful in low power applicationsbecause of this stability.

Electronic ink displays are novel in that they can be addressed by DCvoltages and draw very little current. As such, the conductive leads andelectrodes used to deliver the voltage to electronic ink displays can beof relatively high resistivity. The ability to use resistive conductorssubstantially widens the number and type of materials that can be usedas conductors in electronic ink displays. In particular, the use ofcostly vacuum-sputtered indium tin oxide (ITO) conductors, a standardmaterial in liquid crystal devices, is not required. Aside from costsavings, the replacement of ITO with other materials can providebenefits in appearance, processing capabilities (printed conductors),flexibility, and durability. Additionally, the printed electrodes are incontact only with a solid binder, not with a fluid layer (like liquidcrystals). This means that some conductive materials, which wouldotherwise dissolve or be degraded by contact with liquid crystals, canbe used in an electronic ink application. These include opaque metallicinks for the rear electrode (e.g., silver and graphite inks), as well asconductive transparent inks for either substrate. These conductivecoatings include semiconducting colloids, examples of which are indiumtin oxide and antimony-doped tin oxide. Organic conductors (polymericconductors and molecular organic conductors) also may be used. Polymersinclude, but are not limited to, polyaniline and derivatives,polythiophene and derivatives, poly(3,4-ethylenedioxythiophene) (PEDOT)and derivatives, polypyrrole and derivatives, and polyphenylenevinylene(PPV) and derivatives. Organic molecular conductors include, but are notlimited to, derivatives of naphthalene, phthalocyanine, and pentacene.Polymer layers can be made thinner and more transparent than withtraditional displays because conductivity requirements are not asstringent.

As an example, there are a class of materials called electroconductivepowders which are also useful as coatable transparent conductors inelectronic ink displays. One example is Zelec ECP electroconductivepowders from Du Pont Chemical Co. of Wilmington, Del.

Referring now to FIGS. 1A and 1B, an addressing scheme for controllingparticle-based displays is shown in which electrodes are disposed ononly one side of a display, allowing the display to be rear-addressed.Utilizing only one side of the display for electrodes simplifiesfabrication of displays. For example, if the electrodes are disposed ononly the rear side of a display, both of the electrodes can befabricated using opaque materials, because the electrodes do not need tobe transparent.

FIG. 1A depicts a single capsule 20 of an encapsulated display media. Inbrief overview, the embodiment depicted in FIG. 1A includes a capsule 20containing at least one particle 50 dispersed in a suspending fluid 25.The capsule 20 is addressed by a first electrode 30 and a secondelectrode 40. The first electrode 30 is smaller than the secondelectrode 40. The first electrode 30 and the second electrode 40 may beset to voltage potentials which affect the position of the particles 50in the capsule 20.

The particles 50 represent 0.1% to 20% of the volume enclosed by thecapsule 20. In some embodiments the particles 50 represent 2.5% to 17.5%of the volume enclosed by capsule 20. In preferred embodiments, theparticles 50 represent 5% to 15% of the volume enclosed by the capsule20. In more preferred embodiments the particles 50 represent 9% to 11%of the volume defined by the capsule 20. In general, the volumepercentage of the capsule 20 that the particles 50 represent should beselected so that the particles 50 expose most of the second, largerelectrode 40 when positioned over the first, smaller electrode 30. Asdescribed in detail below, the particles 50 may be colored any one of anumber of colors. The particles 50 may be either positively charged ornegatively charged.

The particles 50 are dispersed in a dispersing fluid 25. The dispersingfluid 25 should have a low dielectric constant. The fluid 25 may beclear, or substantially clear, so that the fluid 25 does not inhibitviewing the particles 50 and the electrodes 30, 40 from position 10. Inother embodiments, the fluid 25 is dyed. In some embodiments thedispersing fluid 25 has a specific gravity matched to the density of theparticles 50. These embodiments can provide a bistable display media,because the particles 50 do not tend to move in certain compositionsabsent an electric field applied via the electrodes 30, 40.

The electrodes 30, 40 should be sized and positioned appropriately sothat together they address the entire capsule 20. There may be exactlyone pair of electrodes 30, 40 per capsule 20, multiple pairs ofelectrodes 30, 40 per capsule 20, or a single pair of electrodes 30, 40may span multiple capsules 20. In the embodiment shown in FIGS. 1A and1B, the capsule 20 has a flattened, rectangular shape. In theseembodiments, the electrodes 30, 40 should address most, or all, of theflattened surface area adjacent the electrodes 30, 40. The smallerelectrode 30 is at most one-half the size of the larger electrode 40. Inpreferred embodiments the smaller electrode is one-quarter the size ofthe larger electrode 40; in more preferred embodiments the smallerelectrode 30 is one-eighth the size of the larger electrode 40. In evenmore preferred embodiments, the smaller electrode 30 is one-sixteenththe size of the larger electrode 40. It should be noted that referenceto “smaller” in connection with the electrode 30 means that theelectrode 30 addresses a smaller amount of the surface area of thecapsule 20, not necessarily that the electrode 30 is physically smallerthan the larger electrode 40. For example, multiple capsules 20 may bepositioned such that less of each capsule 20 is addressed by the“smaller” electrode 30, even though both electrodes 30, 40 are equal insize. It should also be noted that, as shown in FIG. 1C, electrode 30may address only a small corner of a rectangular capsule 20 (shown inphantom view in FIG. 1C), requiring the larger electrode 40 to surroundthe smaller electrode 30 on two sides in order to properly address thecapsule 20. Selection of the percentage volume of the particles 50 andthe electrodes 30, 40 in this manner allow the encapsulated displaymedia to be addressed as described below.

Electrodes may be fabricated from any material capable of conductingelectricity so that electrode 30, 40 may apply an electric field to thecapsule 20. As noted above, the rear-addressed embodiments depicted inFIGS. 1A and 1B allow the electrodes 30, 40 to be fabricated from opaquematerials such as solder paste, copper, copper-clad polyimide, graphiteinks, silver inks and other metal-containing conductive inks.Alternatively, electrodes may be fabricated using transparent materialssuch as indium tin oxide and conductive polymers such as polyaniline orpolythiophenes. Electrodes 30, 40 may be provided with contrastingoptical properties. In some embodiments, one of the electrodes has anoptical property complementary to optical properties of the particles50.

In one embodiment, the capsule 20 contains positively charged blackparticles 50, and a substantially clear suspending fluid 25. The first,smaller electrode 30 is colored black, and is smaller than the secondelectrode 40, which is colored white or is highly reflective. When thesmaller, black electrode 30 is placed at a negative voltage potentialrelative to larger, white electrode 40, the positively-charged particles50 migrate to the smaller, black electrode 30. The effect to a viewer ofthe capsule 20 located at position 10 is a mixture of the larger, whiteelectrode 40 and the smaller, black electrode 30, creating an effectwhich is largely white. Referring to FIG. 1B, when the smaller, blackelectrode 30 is placed at a positive voltage potential relative to thelarger, white electrode 40, particles 50 migrate to the larger, whiteelectrode 40 and the viewer is presented a mixture of the blackparticles 50 covering the larger, white electrode 40 and the smaller,black electrode 30, creating an effect which is largely black. In thismanner the capsule 20 may be addressed to display either a white visualstate or a black visual state.

Other two-color schemes are easily provided by varying the color of thesmaller electrode 30 and the particles 50 or by varying the color of thelarger electrode 40. For example, varying the color of the largerelectrode 40 allows fabrication of a rear-addressed, two-color displayhaving black as one of the colors. Alternatively, varying the color ofthe smaller electrode 30 and the particles 50 allow a rear-addressedtwo-color system to be fabricated having white as one of the colors.Further, it is contemplated that the particles 50 and the smallerelectrode 30 can be different colors. In these embodiments, a two-colordisplay may be fabricated having a second color that is different fromthe color of the smaller electrode 30 and the particles 50. For example,a rear-addressed, orange-white display may be fabricated by providingblue particles 50, a red, smaller electrode 30, and a white (or highlyreflective) larger electrode 40. In general, the optical properties ofthe electrodes 30, 40 and the particles 50 can be independently selectedto provide desired display characteristics. In some embodiments theoptical properties of the dispersing fluid 25 may also be varied, e.g.the fluid 25 may be dyed.

In other embodiments the larger electrode 40 may be reflective insteadof white. In these embodiments, when the particles 50 are moved to thesmaller electrode 30, light reflects off the reflective surface 60associated with the larger electrode 40 and the capsule 20 appears lightin color, e.g. white (see FIG. 2A). When the particles 50 are moved tothe larger electrode 40, the reflecting surface 60 is obscured and thecapsule 20 appears dark (see FIG. 2B) because light is absorbed by theparticles 50 before reaching the reflecting surface 60. The reflectingsurface 60 for the larger electrode 40 may possess retroflectiveproperties, specular reflection properties, diffuse reflectiveproperties or gain reflection properties. In certain embodiments, thereflective surface 60 reflects light with a Lambertian distribution. Thesurface 60 may be provided as a plurality of glass spheres disposed onthe electrode 40, a diffractive reflecting layer such as aholographically formed reflector, a surface patterned to totallyinternally reflect incident light, a brightness-enhancing film, adiffuse reflecting layer, an embossed plastic or metal film, or anyother known reflecting surface. The reflecting surface 60 may beprovided as a separate layer laminated onto the larger electrode 40 orthe reflecting surface 60 may be provided as a unitary part of thelarger electrode 40. In the embodiments depicted by FIGS. 2C and 2D, thereflecting surface may be disposed below the electrodes 30, 40 vis-à-visthe viewpoint 10. In these embodiments, electrode 30 should betransparent so that light may be reflected by surface 60. In otherembodiments, proper switching of the particles may be accomplished witha combination of alternating-current (AC) and direct-current (DC)electric fields and described below in connection with FIGS. 3A-3D.

In still other embodiments, the rear-addressed display previouslydiscussed can be configured to transition between largely transmissiveand largely opaque modes of operation (referred to hereafter as “shuttermode”). Referring back to FIGS. 1A and 1B, in these embodiments thecapsule 20 contains at least one positively-charged particle 50dispersed in a substantially clear dispersing fluid 25. The largerelectrode 40 is transparent and the smaller electrode 30 is opaque. Whenthe smaller, opaque electrode 30 is placed at a negative voltagepotential relative to the larger, transmissive electrode 40, theparticles 50 migrate to the smaller, opaque electrode 30. The effect toa viewer of the capsule 20 located at position 10 is a mixture of thelarger, transparent electrode 40 and the smaller, opaque electrode 30,creating an effect which is largely transparent. Referring to FIG. 1B,when the smaller, opaque electrode 30 is placed at a positive voltagepotential relative to the larger, transparent electrode 40, particles 50migrate to the second electrode 40 and the viewer is presented a mixtureof the opaque particles 50 covering the larger, transparent electrode 40and the smaller, opaque electrode 30, creating an effect which islargely opaque. In this manner, a display formed using the capsulesdepicted in FIGS. 1A and 1B may be switched between transmissive andopaque modes. Such a display can be used to construct a window that canbe rendered opaque. Although FIGS. 1A-2D depict a pair of electrodesassociated with each capsule 20, it should be understood that each pairof electrodes may be associated with more than one capsule 20.

A similar technique may be used in connection with the embodiment ofFIGS. 3A, 3B, 3C, and 3D. Referring to FIG. 3A, a capsule 20 contains atleast one dark or black particle 50 dispersed in a substantially cleardispersing fluid 25. A smaller, opaque electrode 30 and a larger,transparent electrode 40 apply both direct-current (DC) electric fieldsand alternating-current (AC) fields to the capsule 20. A DC field can beapplied to the capsule 20 to cause the particles 50 to migrate towardsthe smaller electrode 30. For example, if the particles 50 arepositively charged, the smaller electrode is placed a voltage that ismore negative than the larger electrode 40. Although FIGS. 3A-3D depictonly one capsule per electrode pair, multiple capsules may be addressedusing the same electrode pair.

The smaller electrode 30 is at most one-half the size of the largerelectrode 40. In preferred embodiments the smaller electrode isone-quarter the size of the larger electrode 40; in more preferredembodiments the smaller electrode 30 is one-eighth the size of thelarger electrode 40. In even more preferred embodiments, the smallerelectrode 30 is one-sixteenth the size of the larger electrode 40.

Causing the particles 50 to migrate to the smaller electrode 30, asdepicted in FIG. 3A, allows incident light to pass through the larger,transparent electrode 40 and be reflected by a reflecting surface 60. Inshutter mode, the reflecting surface 60 is replaced by a translucentlayer, a transparent layer, or a layer is not provided at all, andincident light is allowed to pass through the capsule 20, i.e. thecapsule 20 is transmissive.

Referring now to FIG. 3B, the particles 50 are dispersed into thecapsule 20 by applying an AC field to the capsule 20 via the electrodes30, 40. The particles 50, dispersed into the capsule 20 by the AC field,block incident light from passing through the capsule 20, causing it toappear dark at the viewpoint 10. The embodiment depicted in FIGS. 3A-3Bmay be used in shutter mode by not providing the reflecting surface 60and instead providing a translucent layer, a transparent layer, or nolayer at all. In shutter mode, application of an AC electric fieldcauses the capsule 20 to appear opaque. The transparency of a shuttermode display formed by the apparatus depicted in FIGS. 3A-3D may becontrolled by the number of capsules addressed using DC fields and ACfields. For example, a display in which every other capsule 20 isaddressed using an AC field would appear fifty percent transmissive.

FIGS. 3C and 3D depict an embodiment of the electrode structuredescribed above in which electrodes 30, 40 are on “top” of the capsule20, that is, the electrodes 30, 40 are between the viewpoint 10 and thecapsule 20. In these embodiments, both electrodes 30, 40 should betransparent. Transparent polymers can be fabricated using conductivepolymers, such as polyaniline, polythiophenes, or indium tin oxide.These materials may be made soluble so that electrodes can be fabricatedusing coating techniques such as spin coating, spray coating, meniscuscoating, printing techniques, forward and reverse roll coating and thelike. In these embodiments, light passes through the electrodes 30, 40and is either absorbed by the particles 50, reflected by retroreflectinglayer 60 (when provided), or transmitted throughout the capsule 20 (whenretroreflecting layer 60 is not provided).

The addressing structure depicted in FIGS. 3A-3D may be used withelectrophoretic display media and encapsulated electrophoretic displaymedia. FIGS. 3A-3D depict embodiments in which electrode 30, 40 arestatically attached to the display media. In certain embodiments, theparticles 50 exhibit bistability, that is, they are substantiallymotionless in the absence of a electric field. In these embodiments, theelectrodes 30, 40 may be provided as part of a “stylus” or other devicewhich is scanned over the material to address each capsule or cluster ofcapsules. This mode of addressing particle-based displays will bedescribed in more detail below in connection with FIG. 16.

Referring now to FIGS. 4A and 4B, a capsule 20 of a electronicallyaddressable media is illustrated in which the technique illustratedabove is used with multiple rear-addressing electrodes. The capsule 20contains at least one particle 50 dispersed in a clear suspending fluid25. The capsule 20 is addressed by multiple smaller electrodes 30 andmultiple larger electrodes 40. In these embodiments, the smallerelectrodes 30 should be selected to collectively be at most one-half thesize of the larger electrodes 40. In further embodiments, the smallerelectrodes 30 are collectively one-fourth the size of the largerelectrodes 40. In further embodiments the smaller electrodes 30 arecollectively one-eighth the size of the larger electrodes 40. Inpreferred embodiments, the smaller electrodes 30 are collectivelyone-sixteenth the size of the larger electrodes. Each electrode 30 maybe provided as separate electrodes that are controlled in parallel tocontrol the display. For example, each separate electrode may besubstantially simultaneously set to the same voltage as all otherelectrodes of that size. Alternatively, the electrodes 30, 40 may beinterdigitated to provide the embodiment shown in FIGS. 4A and 4B.

Operation of the rear-addressing electrode structure depicted in FIGS.4A and 4B is similar to that described above. For example, the capsule20 may contain positively charged, black particles 50 dispersed in asubstantially clear suspending fluid 25. The smaller electrodes 30 arecolored black and the larger electrodes 40 are colored white or arehighly reflective. Referring to FIG. 4A, the smaller electrodes 30 areplaced at a negative potential relative to the larger electrodes 40,causing particles 50 migrate within the capsule to the smallerelectrodes 30 and the capsule 20 appears to the viewpoint 10 as a mix ofthe larger, white electrodes 40 and the smaller, black electrodes 30,creating an effect which is largely white. Referring to FIG. 4B, whenthe smaller electrodes 30 are placed at a positive potential relative tothe larger electrodes 40, particles 50 migrate to the larger electrodes40 causing the capsule 20 to display a mix of the larger, whiteelectrodes 40 occluded by the black particles 50 and the smaller, blackelectrodes 30, creating an effect which is largely black. The techniquesdescribed above with respect to the embodiments depicted in FIGS. 1A and1B for producing two-color displays work with equal effectiveness inconnection with these embodiments.

FIGS. 5A and 5B depict an embodiment of a rear-addressing electrodestructure that creates a reflective color display in a manner similar tohalftoning or pointillism. The capsule 20 contains white particles 55dispersed in a clear suspending fluid 25. Electrodes 42, 44, 46, 48 arecolored cyan, magenta, yellow, and white respectively. Referring to FIG.5A, when the colored electrodes 42, 44, 46 are placed at a positivepotential relative to the white electrode 48, negatively-chargedparticles 55 migrate to these three electrodes, causing the capsule 20to present to the viewpoint 10 a mix of the white particles 55 and thewhite electrode 48, creating an effect which is largely white. Referringto FIG. 5B, when electrodes 42, 44, 46 are placed at a negativepotential relative to electrode 48, particles 55 migrate to the whiteelectrode 48, and the eye 10 sees a mix of the white particles 55, thecyan electrode 42, the magenta electrode 44, and the yellow electrode46, creating an effect which is largely black or gray. By addressing theelectrodes, any color can be produced that is possible with asubtractive color process. For example, to cause the capsule 20 todisplay an orange color to the viewpoint 10, the yellow electrode 46 andthe magenta electrode 42 are set to a voltage potential that is morepositive than the voltage potential applied by the cyan electrode 42 andthe white electrode 48. Further, the relative intensities of thesecolors can be controlled by the actual voltage potentials applied to theelectrodes.

In another embodiment, depicted in FIG. 6, a color display is providedby a capsule 20 of size d containing multiple species of particles in aclear, dispersing fluid 25. Each species of particles has differentoptical properties and possess different electrophoretic mobilities (μ)from the other species. In the embodiment depicted in FIG. 6, thecapsule 20 contains red particles 52, blue particles 54, and greenparticles 56, and

|μ_(R)|

|μ_(B)|

|μ_(G)|

That is, the magnitude of the electrophoretic mobility of the redparticles 52, on average, exceeds the electrophoretic mobility of theblue particles 54, on average, and the electrophoretic mobility of theblue particles 54, on average, exceeds the average electrophoreticmobility of the green particles 56. As an example, there may be aspecies of red particle with a zeta potential of 100 millivolts (mV), ablue particle with a zeta potential of 60 mV, and a green particle witha zeta potential of 20 mV. The capsule 20 is placed between twoelectrodes 32, 42 that apply an electric field to the capsule.

FIGS. 7A-7B depict the steps to be taken to address the display shown inFIG. 6 to display a red color to a viewpoint 10. Referring to FIG. 7A,all the particles 52, 54, 56 are attracted to one side of the capsule 20by applying an electric field in one direction. The electric fieldshould be applied to the capsule 20 long enough to attract even the moreslowly moving green particles 56 to the electrode 34. Referring to FIG.7B, the electric field is reversed just long enough to allow the redparticles 52 to migrate towards the electrode 32. The blue particles 54and green particles 56 will also move in the reversed electric field,but they will not move as fast as the red particles 52 and thus will beobscured by the red particles 52. The amount of time for which theapplied electric field must be reversed can be determined from therelative electrophoretic mobilities of the particles, the strength ofthe applied electric field, and the size of the capsule.

FIGS. 8A-8D depict addressing the display element to a blue state. Asshown in FIG. 8A, the particles 52, 54, 56 are initially randomlydispersed in the capsule 20. All the particles 52, 54, 56 are attractedto one side of the capsule 20 by applying an electric field in onedirection (shown in FIG. 8B). Referring to FIG. 8C, the electric fieldis reversed just long enough to allow the red particles 52 and blueparticles 54 to migrate towards the electrode 32. The amount of time forwhich the applied electric field must be reversed can be determined fromthe relative electrophoretic mobilities of the particles, the strengthof the applied electric field, and the size of the capsule. Referring toFIG. 8D, the electric field is then reversed a second time and the redparticles 52, moving faster than the blue particles 54, leave the blueparticles 54 exposed to the viewpoint 10. The amount of time for whichthe applied electric field must be reversed can be determined from therelative electrophoretic mobilities of the particles, the strength ofthe applied electric field, and the size of the capsule.

FIGS. 9A-9C depict the steps to be taken to present a green display tothe viewpoint 10. As shown in FIG. 9A, the particles 52, 54, 56 areinitially distributed randomly in the capsule 20. All the particles 52,54, 56 are attracted to the side of the capsule 20 proximal theviewpoint 10 by applying an electric field in one direction. Theelectric field should be applied to the capsule 20 long enough toattract even the more slowly moving green particles 56 to the electrode32. As shown in FIG. 9C, the electric field is reversed just long enoughto allow the red particles 52 and the blue particles 54 to migratetowards the electrode 54, leaving the slowly-moving green particles 56displayed to the viewpoint. The amount of time for which the appliedelectric field must be reversed can be determined from the relativeelectrophoretic mobilities of the particles, the strength of the appliedelectric field, and the size of the capsule.

In other embodiments, the capsule contains multiple species of particlesand a dyed dispersing fluid that acts as one of the colors. In stillother embodiments, more than three species of particles may be providedhaving additional colors. Although FIGS. 6-9C depict two electrodesassociated with a single capsule, the electrodes may address multiplecapsules or less than a full capsule

In FIG. 10, the rear substrate 100 for a seven segment display is shownthat improves on normal rear electrode structures by providing a meansfor arbitrarily connecting to any electrode section on the rear of thedisplay without the need for conductive trace lines on the surface ofthe patterned substrate or a patterned counter electrode on the front ofthe display. Small conductive vias through the substrate allowconnections to the rear electrode structure. On the back of thesubstrate, these vias are connected to a network of conductors. Thisconductors can be run so as to provide a simple connection to the entiredisplay. For example, segment 112 is connected by via 114 through thesubstrate 116 to conductor 118. A network of conductors may run multipleconnections (not shown) to edge connector 122. This connector can bebuilt into the structure of the conductor such as edge connector 122.Each segment of the rear electrode can be individually addressed easilythrough edge connector 122. A continuous top electrode can be used withthe substrate 116.

The rear electrode structure depicted in FIG. 10 is useful for anydisplay media, but is particularly advantageous for particle-baseddisplays because such displays do not have a defined appearance when notaddressed. The rear electrode should be completely covered in anelectrically conducting material with room only to provide necessaryinsulation of the various electrodes. This is so that the connections onthe rear of the display can be routed with out concern for affecting theappearance of the display. Having a mostly continuous rear electrodepattern assures that the display material is shielded from the rearelectrode wire routing.

In FIG. 11, a 3×3 matrix is shown. Here, matrix segment 124 on a firstside of substrate 116 is connected by via 114 to conductor 118 on asecond side of substrate 116. The conductors 18 run to an edge andterminate in a edge connector 122. Although the display element of FIG.11 shows square segments 124, the segments may be shaped or sized toform a predefined display pattern.

In FIG. 12, a printed circuit board 138 is used as the rear electrodestructure. The front of the printed circuit board 138 has copper pads132 etched in the desired shape. There are plated vias 114 connectingthese electrode pads to an etched wire structure 136 on the rear of theprinted circuit board 138. The wires 136 can be run to one side or therear of the printed circuit board 138 and a connection can be made usinga standard connector such as a surface mount connector or using a flexconnector and anisotropic glue (not shown). Vias may be filled with aconductive substance, such as solder or conductive epoxy, or aninsulating substance, such as epoxy.

Alternatively, a flex circuit such a copper-clad polyimide may be usedfor the rear electrode structure of FIG. 10. Printed circuit board 138may be made of polyimide, which acts both as the flex connector and asthe substrate for the electrode structure. Rather than copper pads 132,electrodes (not shown) may be etched into the copper covering thepolyimide printed circuit board 138. The plated through vias 114 connectthe electrodes etched onto the substrate the rear of the printed circuitboard 138, which may have an etched conductor network thereon (theetched conductor network is similar to the etched wire structure 136).

In FIG. 12, a thin dielectric sheet 150, such as polyester, polyimide,or glass can be used to make a rear electrode structure. Holes 152 arepunched, drilled, abraded, or melted through the sheet where conductivepaths are desired. The front electrode 154 is made of conductive inkprinted using any technique described above. The holes should be sizedand the ink should be selected to have a viscosity so that the ink fillsthe holes. When the back structure 156 is printed, again usingconductive ink, the holes are again filled. By this method, theconnection between the front and back of the substrate is madeautomatically.

In FIG. 14, the rear electrode structure can be made entirely of printedlayers. A conductive layer 166 can be printed onto the back of a displaycomprised of a clear, front electrode 168 and a printable displaymaterial 170. A clear electrode may be fabricated from indium tin oxideor conductive polymers such as polyanilines and polythiophenes. Adielectric coating 176 can be printed leaving areas for vias. Then, theback layer of conductive ink 178 can be printed. If necessary, anadditional layer of conductive ink can be used before the final inkstructure is printed to fill in the holes.

This technique for printing displays can be used to build the rearelectrode structure on a display or to construct two separate layersthat are laminated together to form the display. For example anelectronically active ink may be printed on an indium tin oxideelectrode. Separately, a rear electrode structure as described above canbe printed on a suitable substrate, such as plastic, polymer films, orglass. The electrode structure and the display element can be laminatedto form a display.

Referring now to FIG. 15, a threshold may be introduced into anelectrophoretic display cell by the introduction of a third electrode.One side of the cell is a continuous, transparent electrode 200 (anode).On the other side of the cell, the transparent electrode is patternedinto a set of isolated column electrode strips 210. An insulator 212covers the column electrodes 210, and an electrode layer on top of theinsulator is divided into a set of isolated row electrode strips 230,which are oriented orthogonal to the column electrodes 210. The rowelectrodes 230 are patterned into a dense array of holes, or a grid,beneath which the exposed insulator 212 has been removed, forming amultiplicity of physical and potential wells.

A positively charged particle 50 is loaded into the potential wells byapplying a positive potential (e.g. 30V) to all the column electrodes210 while keeping the row electrodes 230 at a less positive potential(e.g. 15V) and the anode 200 at zero volts. The particle 50 may be aconformable capsule that situates itself into the physical wells of thecontrol grid. The control grid itself may have a rectangularcross-section, or the grid structure may be triangular in profile. Itcan also be a different shape which encourages the microcapsules tosituate in the grid, for example, hemispherical.

The anode 200 is then reset to a positive potential (e.g. 50V). Theparticle will remain in the potential wells due to the potentialdifference in the potential wells: this is called the Hold condition. Toaddress a display element the potential on the column electrodeassociated with that element is reduced, e.g. by a factor of two, andthe potential on the row electrode associated with that element is madeequal to or greater than the potential on the column electrode. Theparticles in this element will then be transported by the electric fielddue to the positive voltage on the anode 200. The potential differencebetween row and column electrodes for the remaining display elements isnow less than half of that in the normal Hold condition. The geometry ofthe potential well structure and voltage levels are chosen such thatthis also constitutes a Hold condition, i.e., no particles will leavethese other display elements and hence there will be no half-selectproblems. This addressing method can select and write any desiredelement in a matrix without affecting the pigment in any other displayelement. A control electrode device can be operated such that the anodeelectrode side of the cell is viewed.

The control grid may be manufactured through any of the processes knownin the art, or by several novel processes described herein. That is,according to traditional practices, the control grid may be constructedwith one or more steps of photolithography and subsequent etching, orthe control grid may be fabricated with a mask and a “sandblasting”technique.

In another embodiment, the control grid is fabricated by an embossingtechnique on a plastic substrate. The grid electrodes may be depositedby vacuum deposition or sputtering, either before or after the embossingstep. In another embodiment, the electrodes are printed onto the gridstructure after it is formed, the electrodes consisting of some kind ofprintable conductive material which need not be clear (e.g. a metal orcarbon-doped polymer, an intrinsically conducting polymer, etc.).

In a preferred embodiment, the control grid is fabricated with a seriesof printing steps. The grid structure is built up in a series of one ormore printed layers after the cathode has been deposited, and the gridelectrode is printed onto the grid structure. There may be additionalinsulator on top of the grid electrode, and there may be multiple gridelectrodes separated by insulator in the grid structure. The gridelectrode may not occupy the entire width of the grid structure, and mayonly occupy a central region of the structure, in order to stay withinreproducible tolerances. In another embodiment, the control grid isfabricated by photoetching away a glass, such as a photostructuralglass.

In an encapsulated electrophoretic image display, an electrophoreticsuspension, such as the ones described previously, is placed insidediscrete compartments that are dispersed in a polymer matrix. Thisresulting material is highly susceptible to an electric field across thethickness of the film. Such a field is normally applied using electrodesattached to either side of the material. However, as described above inconnection with FIGS. 3A-3D, some display media may be addressed bywriting electrostatic charge onto one side of the display material. Theother side normally has a clear or opaque electrode. For example, asheet of encapsulated electrophoretic display media can be addressedwith a head providing DC voltages.

In another implementation, the encapsulated electrophoretic suspensioncan be printed onto an area of a conductive material such as a printedsilver or graphite ink, aluminized Mylar, or any other conductivesurface. This surface which constitutes one electrode of the display canbe set at ground or high voltage. An electrostatic head consisting ofmany electrodes can be passed over the capsules to addressing them.Alternatively, a stylus can be used to address the encapsulatedelectrophoretic suspension.

In another implementation, an electrostatic write head is passed overthe surface of the material. This allows very high resolutionaddressing. Since encapsulated electrophoretic material can be placed onplastic, it is flexible. This allows the material to be passed throughnormal paper handling equipment. Such a system works much like aphotocopier, but with no consumables. The sheet of display materialpasses through the machine and an electrostatic or electrophotographichead addresses the sheet of material.

In another implementation, electrical charge is built up on the surfaceof the encapsulated display material or on a dielectric sheet throughfrictional or triboelectric charging. The charge can built up using anelectrode that is later removed. In another implementation, charge isbuilt up on the surface of the encapsulated display by using a sheet ofpiezoelectric material.

FIG. 16 shows an electrostatically written display. Stylus 300 isconnected to a positive or negative voltage. The head of the stylus 300can be insulated to protect the user. Dielectric layer 302 can be, forexample, a dielectric coating or a film of polymer. In otherembodiments, dielectric layer 302 is not provided and the stylus 300contacts the encapsulated electrophoretic display 304 directly.Substrate 306 is coated with a clear conductive coating such as ITOcoated polyester. The conductive coating is connected to ground. Thedisplay 304 may be viewed from either side.

Microencapsulated displays offer a useful means of creating electronicdisplays, many of which can be coated or printed. There are manyversions of microencapsulated displays, including microencapsulatedelectrophoretic displays. These displays can be made to be highlyreflective, bistable, and low power.

To obtain high resolution displays, it is useful to use some externaladdressing means with the microencapsulated material. This inventiondescribes useful combinations of addressing means with microencapsulatedelectrophoretic materials in order to obtain high resolution displays.

One method of addressing liquid crystal displays is the use ofsilicon-based thin film transistors to form an addressing backplane forthe liquid crystal. For liquid crystal displays, these thin filmtransistors are typically deposited on glass, and are typically madefrom amorphous silicon or polysilicon. Other electronic circuits (suchas drive electronics or logic) are sometimes integrated into theperiphery of the display. An emerging field is the deposition ofamorphous or polysilicon devices onto flexible substrates such as metalfoils or plastic films.

The addressing electronic backplane could incorporate diodes as thenonlinear element, rather than transistors. Diode-based active matrixarrays have been demonstrated as being compatible with liquid crystaldisplays to form high resolution devices.

There are also examples of crystalline silicon transistors being used onglass substrates. Crystalline silicon possesses very high mobilities,and thus can be used to make high performance devices. Presently, themost straightforward way of constructing crystalline silicon devices ison a silicon wafer. For use in many types of liquid crystal displays,the crystalline silicon circuit is constructed on a silicon wafer, andthen transferred to a glass substrate by a “liftoff” process.Alternatively, the silicon transistors can be formed on a silicon wafer,removed via a liftoff process, and then deposited on a flexiblesubstrate such as plastic, metal foil, or paper. As anotherimplementation, the silicon could be formed on a different substratethat is able to tolerate high temperatures (such as glass or metalfoils), lifted off, and transferred to a flexible substrate. As yetanother implementation, the silicon transistors are formed on a siliconwafer, which is then used in whole or in part as one of the substratesfor the display.

The use of silicon-based circuits with liquid crystals is the basis of alarge industry. Nevertheless, these display possess serious drawbacks.Liquid crystal displays are inefficient with light, so that most liquidcrystal displays require some sort of backlighting. Reflective liquidcrystal displays can be constructed, but are typically very dim, due tothe presence of polarizers. Most liquid crystal devices require precisespacing of the cell gap, so that they are not very compatible withflexible substrates. Most liquid crystal displays require a “rubbing”process to align the liquid crystals, which is both difficult to controland has the potential for damaging the TFT array.

The combination of these thin film transistors with microencapsulatedelectrophoretic displays should be even more advantageous than withliquid crystal displays. Thin film transistor arrays similar to thoseused with liquid crystals could also be used with the microencapsulateddisplay medium. As noted above, liquid crystal arrays typically requiresa “rubbing” process to align the liquid crystals, which can cause eithermechanical or static electrical damage to the transistor array. No suchrubbing is needed for microencapsulated displays, improving yields andsimplifying the construction process.

Microencapsulated electrophoretic displays can be highly reflective.This provides an advantage in high-resolution displays, as a backlightis not required for good visibility. Also, a high-resolution display canbe built on opaque substrates, which opens up a range of new materialsfor the deposition of thin film transistor arrays.

Moreover, the encapsulated electrophoretic display is highly compatiblewith flexible substrates. This enables high-resolution TFT displays inwhich the transistors are deposited on flexible substrates like flexibleglass, plastics, or metal foils. The flexible substrate used with anytype of thin film transistor or other nonlinear element need not be asingle sheet of glass, plastic, metal foil, though. Instead, it could beconstructed of paper. Alternatively, it could be constructed of a wovenmaterial. Alternatively, it could be a composite or layered combinationof these materials.

As in liquid crystal displays, external logic or drive circuitry can bebuilt on the same substrate as the thin film transistor switches.

In another implementation, the addressing electronic backplane couldincorporate diodes as the nonlinear element, rather than transistors.

In another implementation, it is possible to form transistors on asilicon wafer, dice the transistors, and place them in a large areaarray to form a large, TFT-addressed display medium. One example of thisconcept is to form mechanical impressions in the receiving substrate,and then cover the substrate with a slurry or other form of thetransistors. With agitation, the transistors will fall into theimpressions, where they can be bonded and incorporated into the devicecircuitry. The receiving substrate could be glass, plastic, or othernonconductive material. In this way, the economy of creating transistorsusing standard processing methods can be used to create large-areadisplays without the need for large area silicon processing equipment.

The following Example is now given, though by way of illustration only,to show details of particularly preferred reagents, conditions andtechniques used in the electrophoretic media and displays of the presentinvention.

EXAMPLE

An internal phase was prepared comprising 10 percent by volume whiteparticles and 1 percent by volume black particles (carbon black) byvolume in a hydrocarbon suspending fluid; the internal phase had aviscosity of 4.75 mPa sec. The white particles comprised titania and hadan average size of approximately 0.6 μm and a saturation particlethickness estimated at 1.5 to 2.5 μm. The internal phase wasencapsulated in gelatin/acacia microcapsules substantially as describedin Paragraphs [0069] to [0074] of U.S. Patent Publication No.2002/0180687. The resultant microcapsules were separated into threebatches differing in wet capsule diameter size distributions. Each batchwas mixed into a slurry with a polymeric binder, coated to form anelectrophoretic film, and laminated to a back electrode to form aswitchable display pixel, substantially as described in Paragraphs[0075] and [0076] of the aforementioned 2002/0180687. During the coatingprocess, suitable equipment settings such as speed, pressure and dieheight were used to achieve a range of wet film coat weights, which thendried into capsules of differing IP heights in part due to the effectsof binder evaporation and surface tension. In one batch, the drycapsules were roughly spherical; in the second batch the dry capsuleshad substantially the form of oblate spheroids; and in the third batchthe dry capsules had substantially the form of prolate spheroids, withheights greater than their diameters. In each case the spheroids rangedfrom circular in XY projection to hexagonal, varying with the packingdensity in the film of electrophoretic medium.

The three resultant electrophoretic media differed in estimated IPheights and typical pixel optical properties when switched with a 350 ms15 V pulse, as shown in the Table below.

TABLE Estimated IP Maximum Height in White Dark Contrast VisualMicrocavity (μm) State L* State L* Ratio Artifacts L* 18 62 21 9.4:1 1.544 59 27 5.3:1 3.7 55 60 24 6.9:1 5.5

Assuming a saturation particle thickness for the titania of 2 μm, theoptimum IP height according to the present invention should be 20 μm;the IP height calculated for the black pigment is substantially less, sothat it is the optimum IP height for the titania which is important forthis medium. It will be seen from the data in the Table above that thecapsules having an estimated IP height of 18 μm, close to the calculated20 μm, had substantially better optical properties, including animproved contrast ratio, as compared with the other two media havingsubstantially greater IP heights.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense, theinvention being defined solely by the appended claims.

1. An electrophoretic medium having walls defining at least onemicrocavity containing an internal phase, this internal phase comprisinga plurality of at least one type of electrophoretic particle suspendedin a suspending fluid and capable of moving therethrough uponapplication of an electric field to the electrophoretic medium, theaverage height of the at least one microcavity differing by not morethan about 5 μm from the saturated particle thickness of theelectrophoretic particle divided by the volume fraction of theelectrophoretic particles in the internal phase.
 2. An electrophoreticmedium according to claim 1 wherein the saturated particle thickness isbetween about 1 and about 5 μm.
 3. An electrophoretic medium accordingto claim 2 wherein the saturated particle thickness is between about 1.5and about 2.5 μm.
 4. An electrophoretic medium according to claim 1 suchthat, if application of a specific electric field to the medium for timeT suffices to switch the medium between its extreme optical states,variations in the time of application of this specific electric fieldwithin the range of 0.9 to 1.1 T will not change the optical propertiesof either extreme state of the electrophoretic medium by more than 2units of L*.
 5. An electrophoretic medium according to claim 4 such thatvariations in the time of application of this specific electric fieldwithin the range of 0.8 to 1.2 T will not change the optical propertiesof either extreme state of the electrophoretic medium by more than 2units of L*.
 6. A electrophoretic medium according to claim 1 comprisinga single type of electrophoretic particle in a colored suspending fluid.7. A electrophoretic medium according to claim 1 comprising a first typeof electrophoretic particle having a first optical characteristic and afirst electrophoretic mobility and a second type of electrophoreticparticle having a second optical characteristic different from the firstoptical characteristic and a second electrophoretic mobility differentfrom the first electrophoretic mobility.
 8. A electrophoretic mediumaccording to claim 7 wherein the suspending fluid is uncolored.
 9. Aelectrophoretic medium according to claim 1 wherein the electrophoreticparticles and the suspending fluid are retained within a plurality ofcavities formed within a carrier medium.
 10. A electrophoretic mediumaccording to claim 1 wherein the electrophoretic particles and thesuspending fluid are held within a plurality of capsules.
 11. Anelectrophoretic medium according to claim 1 wherein the electrophoreticparticles comprise titania.
 12. An electrophoretic medium according toclaim 11 wherein the electrophoretic particles further comprise darkcolored particles formed from carbon black or copper chromite, the darkcolored particles formed from carbon black or copper chromite and havingan electrophoretic mobility different from the electrophoretic mobilityof the titania particles.
 13. An electrophoretic medium according toclaim 1 wherein the volume fraction of electrophoretic particles in theinternal phase is from about 3 to about 40 percent.
 14. Anelectrophoretic medium according to claim 13 wherein the volume fractionof electrophoretic particles in the internal phase is from about 6 toabout 18 percent.
 15. An electrophoretic medium according to claim 1having an internal phase height between about 10 and about 30 μm and avolume fraction of electrophoretic particles of between about 3 andabout 15 percent.
 16. An electrophoretic medium according to claim 15having an internal phase height between about 12 and about 25 μm and avolume fraction of electrophoretic particles of between about 5 andabout 12 percent.
 17. An electrophoretic medium according to claim 1wherein the viscosity of the internal phase is less than about 5 mPasec.
 18. An electrophoretic medium according to claim 17 wherein theviscosity of the internal phase is greater than about 1 mPa sec.
 19. Anelectrophoretic display comprising an electrophoretic medium accordingto claim 1 and at least one electrode disposed adjacent theelectrophoretic medium and arranged to apply an electric field thereto.20. An electrophoretic display according to claim 19 having a firstoptical state in which the display displays an optical characteristic ofthe one type of electrophoretic particle and a second optical state inwhich the electrophoretic medium is light-transmissive.
 21. Anelectrophoretic display according to claim 20 wherein, in thelight-transmissive optical state, the electrophoretic particles areconfined in a minor proportion of the cross-sectional area of eachmicrocavity.
 22. An electrophoretic display according to claim 20comprising a backplane comprising a plurality of pixel electrodes, and acolor filter or reflector, the color filter or reflector being disposedbetween the backplane and the electrophoretic medium.